This invention relates to a reroute strategy for a multilayered electrical interconnect.
A conventional approach for rerouting interconnect lines is to first remove the electrical component's inside lead pads (i.e., bonding pads placed on the electrical component or integrated wire circuit) selected interconnect lines that are fabricated within an underlying interconnect substrate. To remove an interconnect line, a wire bond must be broken at the inside lead pad. After the defective wire is removed, a discrete jumper wire is then attached to the selected inside lead pad. Jumper wires often must travel long distances between electrical components placed on opposite ends of the substrate. If extensive rerouting is required, numerous jumper wires must be attached at target locations throughout the substrate material. If the target connections are concentrated within a small area, discrete wires of large diameter cannot be used. Conversely, if small wires are used, they often prove too fragile to handle. With the advent of very high density interconnects (e.g., line densities exceeding 500 lines/inch/layer) the entire strategy of jumper wire rerouting must be revised. Extensive rerouting cannot be achieved in high density interconnects using conventional jumper wire techniques.
Recently, attempts have been made to place the reroute lines within the interconnect substrate. Instead of using long jumper wires extending between inside lead pads, more recent prior art places part of the reroute line within a multilayered substrate. The embedded interconnect lines are routed through vias to dumbell-shaped pads placed on the surface layer. The dumbell shaped pads are then connected to outside lead pads (i.e., bonding pads placed on the substrate near each electrical component) by short fly-wires. A short wire lead then must connect each outside lead pad to a corresponding inside lead pad. Thus, although part of the interconnect line is placed within a substrate, in order to connect the embedded interconnect lines to selected inside lead pads, both a fly wire and a wire lead are needed. If the fly wires and/or wire leads are long, undesirable impedance affects the system causing signal delays and cross-talk problems. Furthermore, since both the fly wires and wire leads require soldering, there is an increased risk of dislodging the electrical component upon impact causing a less reliable interconnect system. Also, if fly-wires or wire leads are routed across one another, there exists a potential for shorting.
An associated problem inherent with placing reroute lines within the interconnect within the substrate is the inefficient use of the interconnect. Instead of seeking the shortest distance from point A to point B, typical fabricated interconnect layers often route from point A to point B via points C, D, etc., which may be distant from the target area. The interconnect line, for example, from point A to point C and back to point B may be unacceptably long, causing problems similar to those associated with long jumper wires. Often, interconnect lines extend the entire length of the substrate and are not severable at desired locations near the target location. When a connection is made at the target area, the unused interconnect portion extends beyond the link location and forms an unusable stub residing within the substrate. The existence of nonuseable stubs decreases available interconnect lines, thereby forcing a reduction in interconnect density.